1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same. More specifically, the invention relates to a submicron MOSFET (Metal Oxide Semiconductor Field Effect Transistor) having so-called source-to-drain nonuniformly doped channel (NUDC) constitution where a channel region doped with an impurity of high concentration and another channel region doped with an impurity of low concentration are distributed between a source and drain of a MOS transistor, and a method for fabricating the same.
2. Related Art
Methods for forming a semiconductor device with the NUDC constitution have heretofore been reported, for example, by Y. Okumura et al. [see "A Novel Source-to-Drain Nonuniformly Doped Channel (NUDC) MOSFET for High Current Drivability and Threshold Voltage Controllability", pp. 391-394, IDEM90]. The method of an NUDC MOSFET fabrication disclosed by Y. Okumura is explained hereinbelow with reference to FIGS. 6(a), 6(b), 6(c) and 6(d).
Referring initially to FIG. 6(a), a gate oxide 2 is formed on a p-type silicon substrate 1, and a polysilicon layer 3 doped with phosphorus is grown thereon by a Chemical Vapor Deposition (CVD) process.
As shown in FIG. 6(b), a gate electrode 3a is formed into a predetermined pattern by well-known photo-lithography and etching processes. Oblique ion implantation of boron 4 at an incidence angle .theta. of 20.degree.-40.degree. and rotating in a direction A is performed to form a high concentration p-type NUDC layer 4a.
Referring to FIG. 6(c), after ion implantation of phosphorus 5 from just above, i.e, at an incidence angle of 0.degree., a low concentration n.sup.- -type layer 5a having lightly doped drain (LDD) constitution is provided on the above layer 4a. FIG. 6(d) shows that a side wall 7 is formed outside the gate electrode 3a. Oblique ion implantation of boron 6 at an incidence angle of 7.degree. is carried out to form a high concentration n-type layer 6a as a source and a drain.
In the submicron n-type NUDC MOSFET fabricated in a manner as described above, the high concentration p-type layer 4a can prevent punch-through phenomenon between the source and drain so that short channel effect can also be suppressed.
Nevertheless, a disadvantage exists in the above prior art method in which before the formation of the source and drain region, doping of the high concentration p-type impurity is previously performed in a site intended to be the region. Thus, bonding pressure between the high concentration n-type impurity doped layer 6a and the high concentration p-type layer 4a is considerably decreased, facilitating current crowding effect. The result is that hot carrier effect is undesirably caused.